The production of three-dimensional articles of complex shape by means of stereolithography has been known for a number of years. In this technique the desired shaped article is built up from a radiation-curable composition with the aid of a recurring, alternating sequence of two steps (a) and (b). In step (a), a layer of the radiation-curable composition, one boundary of which is the surface of the composition, is cured with the aid of appropriate imaging radiation, preferably imaging radiation from a computer-controlled scanning laser beam, within a surface region which corresponds to the desired cross-sectional area of the shaped article to be formed, and in step (b) the cured layer is covered with a new layer of the radiation-curable composition, and the sequence of steps (a) and (b) is repeated until a so-called green model of the desired shape is finished. This green model is, in general, not yet fully cured and may therefore be subjected to post-curing, though such post curing is not required.
Via an equivalent process, photopolymer can be jetted by ink jet or multiple ink jet processes in an imagewise fashion. While jetting the photopolymer or after the photopolymer is applied actinic exposure can be provided to initiate polymerization. Multiple materials (for example non-reactive waxes, weakly reacting photopolymers, photopolymers of various physical properties, photopolymers with various colors or color formers, etc.) can be jetted or applied to provide supports or alternate cured properties.
The mechanical strength of the green model (modulus of elasticity, fracture strength), also referred to as green strength, constitutes an important property of the green model and is determined essentially by the nature of the stereolithographic-resin composition employed in combination with the type of stereolithography apparatus used and degree of exposure provided during part fabrication. Other important properties of a stereolithographic-resin composition include a high sensitivity for the radiation employed in the course of curing and a minimum amount of curl or shrinkage deformation, permitting high shape definition of the green model. In addition, for example, it should be relatively easy to coat a new layer of the stereolithographic resin composition during the process. And of course, not only the green model but also the final cured article should have optimum mechanical properties.
The developments in this area of technology move towards compositions having better mechanical properties in order to better simulate properties of commodity materials like polypropylene and engineering type polymers. Also there exists a requirement for faster cure and process speeds, so as to decrease the time to built a part. This has resulted in new stereolithography machines having solid state lasers that have a high energy output, very fast laser-scanning and faster recoating processes. The new machines supply UV light with a power around 800 mW and above, compared to 200–300 mW for the older conventional machines. Also the scanning time is reduced by 3 to 4 times. These high powers, high scanning speeds, and short recoating times result in higher temperatures, due to polymerization exotherm of the resins and parts during fabrication. Typical temperatures have risen to values between 50 and 90° C., which leads to part distortion and excessive color development.
The developments have resulted in compositions having a substantial amount of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, together with other components like polyols, acrylates and glycidylethers. Examples of such compositions can be found in for example U.S. Pat. No. 5,476,748. A disadvantage of such compositions is that cured parts show poor mechanical properties, when subjected to high humidity environments or when they are soaked in water for longer periods of time.
JP Hei 11-199647 shows examples of hybrid compositions, containing oxetanes, 38–50 wt % 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate and more than 35 wt % of acrylates. These compositions have the disadvantage that the green model parts show unacceptable high distortion when made in a high power solid state stereolithography apparatus, whereby the parts also are very sensitive to water or high humidity.